1) Field of the Invention
The present invention relates to an image fixing device that is employed in a multi-function xerograph that serves as a facsimile device, printer, as well as a copier. The present invention also relates to an image forming device that employs the image fixing device.
2) Description of the Related Art
FIG. 3 is a side view of the internal structure of a conventional copier and facsimile device that includes an image forming device that in turn includes an image fixing device. This facsimile device has a main body 1. This main body 1 consists of a removable process cartridge 2 that comprises various process devices for forming color images by xerography. A photoreceptor 3 is provided in the process cartridge 2. There is provided an optical writing device 4 that causes the photoreceptor 3 to scan according to the laser beam that is modulated based on the image data. A paper feed cassette 5 houses the paper (i.e., recording paper) on which the image is to be printed (i.e., recorded). A paper feed roller 6 feeds the paper housed in the paper feed cassette 5. A transfer roller 7 transfers the toner image formed on the recording paper. There is provided an image fixing device 8 that consists of a fixing roller 9 and a pressure roller 10. The image fixing device 8 fixes the toner image to the paper. There are also provided a paper ejection roller 11 that ejects the paper to the outside of the main body 1. There are also provided a contact sensor 12. A draft tray 13 is provided on top of the main body 1 on which a draft that is to be faxed is placed.
Although not shown in FIG. 1, the copier and facsimile device has a communication device that receives image data from other facsimile devices or sends image data to other facsimile devices via a telephone line or the like.
The draft placed on the draft tray 13 is made to go past the contact sensor 12 by a conveyer system and ejected outside of the main body 1. When the draft goes past the contact sensor 12, the image on the draft is optically read by the contact sensor 12. The image data read by the contact sensor 12 or the image data received by the communications device is sent to the optical writing device 4. The optical writing device 4 irradiates the surface of an uniformly charged photoreceptor 3 with a laser beam modulated according to the image data. This causes an electrostatic latent image to be formed on the surface of the photoreceptor 3. The electrostatic latent image is converted to a toner image by fixing the toner. The transfer roller 7 transfers the toner image to the recording paper. The recording papers is then made to pass through the image fixing device 8 where the toner image is fixed to the recording paper by applying heat and pressure. Precisely, recording papers is made to pass through a nipping part formed by the fixing roller 9 and the pressure roller 10image fixing device. The paper ejecting roller 11 ejects the recording paper to the outside of the main body 1.
It is very common to include a fixing roller and a pressure roller in the image fixing device, and provide a fixing heater such as an infra-red heater or a halogen heater in both the fixing roller and the pressure roller. However, the shaft of the pressure roller has a layer of silicon gum, and a parting layer such as a Teflon (registered trademark) tube is laminated above the layer of silicon gum. Therefore, even if the pressure roller has the heater inside it, it does not get heated up as desired. This causes inadequate fixing. There is an added disadvantage in terms of material cost which goes up considerably. Also, high pressure application is required in order to secure a moderate nipping width between the fixing roller and the pressure roller. If pressure higher than the appropriate pressure is applied, it results in stress on the recording paper and causes wrinkles.
There has been research along the lines of fabricating a image fixing device in which, instead of employing a pressure roller, the nipping part is created by pressing a pressure plate against the fixing roller. By virtue of the flat shape of the pressure plate, the heat application is more effective than when the pressure roller is used. This also made the securing of appropriate nipping width possible.
However, it is desirable that the following conditions are satisfied if a pressure plate is employed. These are, the pressure plate should have a low coefficient of friction and should not apply extra load to the paper while the paper is transported, should have a good heat resistance and low thermal expansion, and should have low abrasion loss. One material that has a high heat resistance and low coefficient of friction is fluorocarbon resin. However, the polytetraflouroethylene (PTFE) in the fluorocarbon resin is inferior to PFA or FEP in mechanical strength, even though PTFE has a lower coefficient of friction than PFA and FEP.
Further, in the conventional image fixing device with the fixing roller and the pressure roller, the force on the paper during conveying can be adjusted as desired. However, this is not possible in the image fixing device with the pressure plate. The result can be jamming of the paper between the fixing roller and the pressure plate.
One solution for reducing the possibility of jamming that was considered was to use PTFE as the pressure plate. However, there does exist friction between the pressure plate and the fixing roller or the paper. This wears the pressure plate and when the abrasion loss becomes sufficiently large, causes tears in the plate, resulting in inadequate fixing. It is also required that the pressure plate should have mechanical strength in order for it to maintain its position against the fixing roller to secure the nipping width or to avoid or not to be damaged when a jam is being released.
It is an object of the present invention to provide a image fixing device in which the abrasion loss due to friction between the pressure plate and the fixing roller or the paper is substantially reduced.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.